Known solar collector assemblies include a box-type bottom portion made of sheet metal and having an upper aperture closed by a glass plate. An absorber is disposed in the sheet metal housing. An insulating material such as foamed styrene, glass wool or the like is located between the absorber and the bottom of the sheet metal housing. Such collectors are referred to as mono-cover single collectors. These prior art collectors are not well suited for dwelling houses because of the considerable height resulting from the inclusion of the insulating packages required to prevent heat loss in a downward direction. It is not possible to reduce the height of the prior art structure without sacrificing the insulating effect. Furthermore, this type of collector has a relatively large weight constituting a further disadvantage. Moreover, the high edge of the box-type sheet metal housing throws shadows over a large part of the absorber inside the housing. Thus, the efficiency of the absorber is reduced and the output of the collector is insufficient when an angular incidence of the sun rays occurs to produce these shadows. With the prior art collectors, the sun energy is effectively converted into heat only when there is a relatively vertical incidence of sun radiation. Without insulation on the upper side of the collector, there is a further restrictive effect upon the efficiency and output of the prior art solar collector assemblies.
The known double cover solar collector is identical to the mono-cover single collector with respect to the box-type sheet metal housing and the required thick insulating package. In this collector, however, two glass covers are superimposed in spaced relationship with respect to each other forming an air cushion between them. This configuration reduces the amount of thermal loss of the absorber in upward direction to obtain a heat storage in the absorber when solar radiation decreases. The double cover collector involves the same disadvantages in its structure and use as the mono-cover single collector.
The solar collector disclosed in U.S. Pat. No. 3,916,871 discloses the use of a vacuum pump connected to the hollow body containing the absorber. The vacuum pump is used to evacuate the hollow body from time to time and maintain a constant pressure level. The vacuum in the hollow body insulates the absorber to thereby eliminate the necessity for insulating packages thus permitting the construction of the solar collector with a more compact structural configuration.